The present invention relates generally to electronic angle sensing devices. More particularly, this invention pertains to electronic vertical angle sensing and indicating devices for use on aiming systems associated with projectile launchers.
Archers have relied for many years upon pin sights for aiming their bows. Typically, these pins are adjusted to accurately direct an arrow trajectory toward a target at a specific distance from the archer. Each pin is set for a specific distance, for example, 20, 30, and 40 yards. It is assumed by the archer that, while setting and aligning the pins on his sight, the bow riser (the frame of the bow that includes the handle) is vertical and that the arrow is launched with only forward and upward components of velocity. If the riser is canted to the left or right, i.e., tilted about a horizontal axis directed toward a target, a small sideway component of velocity is imparted to the arrow, resulting in a left or right transverse horizontal drift, respectively, as it flies toward its target. Referring now to FIG. 1, an illustration of a bow aimed at a target shows the target direction and trajectory of an arrow in relation to the coordinate axes used to describe the effect of tilting the bow in the plane perpendicular to the target direction. Referring now to FIG. 2 a detailed illustration of the components of a bow relevant to the problem of bow canting shows the positioning of a bow sight-mounted level sensor on the bow.
Prior developments in this area of angle sensing devices for use on aiming systems associated with projectile launchers include bubble-type levels, similar to those used on carpenters levels, and mechanical pendulum devices. Typically, if an archer wants to ensure that the bow riser is vertical, a small bubble-type level is attached horizontally to the base of a pin sight housing within the field of view of the peep sight attached to the string. When the bubble-type level indicates that the base of the pin sight housing is level, the riser of the bow will have not be canted and will have a zero cant angle (i.e. a cant angle of 0°). The condition of zero cant angle will be referred to as being in “vertical level”. While bubble-type levels perform reasonably well for short trajectory shooting situations, they are somewhat limited when the arrow trajectory is long.
Canting is also of concern when firing a rifle, grenade launcher, or any other projectile-firing device. The effect of canting angle on the accuracy of a projectile trajectory can be better understood by examining the geometry associated with the launch of a projectile. Consider a coordinate system in which the x-axis is defined as the forward-looking horizontal line from the projectile launcher to the target, the y-axis is the vertical direction, and the z-axis is directed horizontally to the right of the launcher. A projectile is normally launched in the x-y plane with a launch angle, Θ, above the x-axis. The cant angle, Φ, represents the tilt in the y-z plane about the x-axis. When the projectile launcher is canted about the x-axis, the projectile is given a small component of motion along the z-axis, which causes the projectile to drift left or right of the intended target. Simultaneously, the cant angle causes the launch angle to be reduced slightly, causing the projectile the fall short.
Aiming systems using either bubble-type levels or mechanical pendulum devices are bulky and cumbersome when used with portable projectile launchers such as bows and firearms. For most projectile launchers, it is desirable that the weight of any accessory be kept to a minimum. This is especially true when an angle sensing device is attached to bowsights, because of the high amount of shock and vibration generated during the process of shooting of an arrow. The construction of typical bowsights requires that the electronic angle sensing device be mounted on a cantilevered beam of some type, which is in turn attached to the bow riser. Excessive mass attached to such a bowsight can cause extreme stresses on the sight frame, eventually causing the bowsight attachment point to fatigue and fracture.
What is needed, then, is a lightweight vertical level sensing and indicating system that can be attached directly to the housing of a bowsight.
Recent developments in the area of electronic angle sensing devices for use on aiming systems associated with projectile launchers include incorporating a “binary” (i.e. in vertical level or not in vertical level) indication of vertical level with the signal indicator housed on the eyepiece. One such binary device locates the signal indicator in the peripheral field-of-view out of the focal plane of the reticle. This requires the user to view the signal indicators to rely on peripheral vision and presents a blurred near-field illumination. While providing a binary indication of vertical level, these devices do not provide for analog or digital output signals that are proportional to the cant angle.
What is needed, then, is a small, lightweight, affordable vertical level indicator that incorporates a miniaturized, low-power electronic angle sensing device to provide an output signal that is proportional to the cant angle for use with aiming systems associated with projectile launchers.